We are continuing our studies of partial coronary artery obstruction in a canine model. We have shown that platelet aggregates can form in a stenosed coronary artery, thus cutting off the coronary flow and leading to sudden death of the dog. We are studying aspirin, sulfinpyrazone and other agents which appear to protect against this platelet plugging. We are also studying an apparent "coronary steal" where blood flow is preferentially taken away from the partially obstructed coronary and given to the other normal beds by vasodilators such as persantin, and nitroglycerin. These studies suggest that vasodilators may do more harm than good under certain circumstances.